In optical fiber communications, enlargement of a transmission distance and increase in a transmission capacity have been achieved by using an erbium doped optical fiber amplifier (EDFA). Recently, in addition to the EDFA, a Raman amplifier has been utilized to enlarge a transmission distance, to increase a capacity and to increase an output, etc.
The Raman amplifier as described above, and As disclosed in, for example, Japan Patent No. 5074645 and Japanese Patent Application Laid-open Publication No. 2002-50828 is used which includes a semiconductor laser device having a semiconductor laser element and a fiber Bragg Grating (FBG) and an optical fiber into which a laser light as a pumping light outputted from this semiconductor laser device is inputted and which makes a signal light be subjected to Raman amplification. Herein the semiconductor laser element includes a semiconductor light-emitting unit provided with an active layer, and an optical cavity having a first reflection unit (reflection layer) reflecting a light outputted by this semiconductor light-emitting unit.
In some cases, a laser light outputted by the semiconductor laser having the above-described optical cavity may be temporarily in a single-mode oscillation state in a spectrum area (longitudinal mode) at a low output side, for example, equal to or smaller than 50 mW. At a time of such single-mode oscillation, a problem will occur that an optical intensity per one longitudinal mode increases, stimulated Brillouin scattering (SBS) occurs by a non-linear optical effect in the optical fiber, and thus a stimulated Brillouin scattering light propagates as an optical feedback in the direction opposite to the light outputted by the semiconductor laser. This optical feedback may become a factor hindering stable operation of an optical communication system.
There is a need for a method of designing a semiconductor laser device capable of restraining stimulated Brillouin scattering in an optical fiber caused by a laser light outputted by a semiconductor laser device used in a pumping light source for Raman amplification, a method of designing a Raman amplifier, a method of manufacturing a semiconductor laser device, a semiconductor laser device, a Raman amplifier, and an optical communication system.